Information communication efficiency over an optical fiber transmission system may be increased by optical wavelength division multiplexing. Wavelength division multiplexed (WDM) systems employ WDM signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information on optical fiber. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM signals.
To facilitate the addition and subtraction of particular carrier signals to or from the WDM signal at different locations on the network, optical add/drop filters are employed. It should be noted that channels within a WDM signal may be distinguished by either their optical wavelength or optical frequency, and the terms frequency and wavelength will be interchangeably referred to for this purpose.
One known type of network employing such filters is the optical ring network. In a ring network, a ring of one or more transmission fibers facilitates communication between several network nodes. In such networks, a node may originate an information signal which is then modulated onto a carrier signal having a certain optical wavelength. The modulated optical signal is subsequently added to the WDM signal for transmission on the ring network to the appropriate destination node or nodes. At the section of the WDM ring located near the destination node, a filter removes or drops the appropriate channel from the WDM signal. The dropped channel or optical signal is then transmitted to the destination node, where the information signal may be demodulated and used.
For further explanation of the details and operations of such networks, see, for example, A. F. Elrefaie, "Multiwavelength Survivable Ring Network Architectures," GLOBECOM '89 Proc., Dallas, Tex. (Nov. 1989), and A. F. Elrefaie, et al., "Fiber-Amplifier Cascades with Gain Equalization in Multiwavelength Unidirectional Inter-Office Ring Networks," IEEE Photonics Tech. Letters, Vol. 5, No. 9, pp. 1026-28 (September 1993), both of which are incorporated by reference herein. Optical filters capable of dropping and adding signals at specific wavelengths into an optical fiber are essential components in any network utilizing WDM signals including such ring networks.
At present, add/drop filters ordinarily consist of a 1.times.N demultiplexer followed by an N.times.1 multiplexer. The first element demultiplexes the frequency components of the input WDM signal, providing the frequency components to its N output ports. A desired frequency component is then dropped by connecting the corresponding output port to a drop line fiber that is further connected to a destination node. The filter's N.times.1 multiplexer remultiplexes the remaining N-1 frequency components or channels, providing the WDM signal to the multiplexer output port. A signal of the same frequency as that of the dropped component may be added to the WDM signal using the unused port of the multiplexer.
One drawback of such filters is that the channel that is added and/or dropped is fixed by the physical configuration of the filter. Once an output port of the N.times.1 demultiplexer has been coupled to the drop line fiber, only that coupled channel may be dropped. Such inflexibility is undesirable.
Another prior art filter attempts to overcome this inflexibility by providing an acoustooptic tunable add/drop optical filter. K.-W. Cheung, "Acoustooptic Tunable Filters in Narrowband WDM Networks: System Issues and Network Applications," Journal on Selected Areas in Communications, Vol. 8, No. 6, pp. 1015-25 (August 1990). In this filter, the frequency component that is dropped may be controllably selected without physically reconfiguring the fiber connections.
The acoustooptic filter includes a waveguide substrate having an input port, a grating, an output port and a drop port. The filter also includes a source of acoustic waves which provides an acoustic wave to the grating of the substrate. The acoustic wave causes a particular optical frequency from an input WDM signal to propagate toward the drop port while the remaining optical frequencies propagate towards the output port. The optical frequency which is removed depends upon the frequency of the acoustic wave applied to the grating. Thus, control of the acoustic source allows the filter to be controllably tuned. Wavelength selective space-division optical networks employing this drop/add filter are proposed in the Cheung article, referred to above, at pp. 1022-25. However, it is noteworthy that such acoustooptic tunable filters have the added requirement of a finely-tuned source of acoustic waves.